A voltage-source inverter includes an AC/DC converter supplying a DC voltage to the inverter through a DC-link including a capacitor and an inductance, as generally known. The AC/DC converter and the inverter are both generally in the form of a bridge including static power switches connected across the DC terminals and the AC lines.
Voltage-source inverters with a fixed voltage DC-link are increasingly being used in high performance motor drive applications. Typically, such motor drives operate through the four quadrants in terms of positive, or negative, current and of motoring, or regenerating, mode. However, the DC-link is commonly supplied from a single-quadrant rectifier bridge, i.e. not accepting energy returned from the motor when braking, thus not returned to the AC power supply.
In a modern voltage-source inverter controlled motor drive, the drive is capable of operating in two quadrants of the speed-torque plane. The first quadrant is where both speed and torque are positive. The third quadrant is where both speed and torque are negative. Thus, in those two instances the product (torque.times.speed) is positive. This is when power flow goes into the mechanical load of the motor. In contrast, the second and fourth quadrants are those where the product (torque.times.speed) is negative, i.e. when the motor is acting as a generator and power is flowing from the mechanical load through the motor and back into the inverter side of the drive.
The purpose is to obtain a voltage-source inverter that can accept return power flow, i.e. the power can flow back through the inverter to the DC-link capacitor. Where the problem lies is designing an AC/DC converter that can accommodate negative load current. In other words, to a single-quadrant rectifier bridge should be substituted a two-quadrant power converter.
The prior art shows power circuits and control schemes which can be used to obtain two-quadrant power conversion in a voltage-source inverter motor drive. See for instance U.S. Pat. Nos. 4,353,023 and 4,434,393 using an antiparallel regenerative thyristor bridge circuit connected between the inverter and the rectifier through switching transistors fired in synchronism with the thyristors. This approach requires at least six additional thrysitors and associated control, which is a costly solution. A more simple solution has been proposed to deal with the negative flow of power into the DC-link from the inverter, consisting in using a resistor-chopper. This solution, however, because it employs a resistor, is cost effective, but has an adverse impact on efficiency and it causes temperature to rise.